Crop mixtures in traditional systems

The traditional cropping system is stable because it is adapted to the
farmers' level of technology and the soils' capability. It incorporates mixed
cropping and bush fallow, and it gives a high total return per unit area of
land. Furthermore, growing crops in mixtures is consistent with the farmers'
goal of security. Their present systems have evolved naturally as an answer to
the challenging environment in which they live.

Researchers have been hesitant to tackle multiple cropping experiments in
general, and agro-forestry in particular, because of the infinite combinations
possible, lack of knowledge about existing systems, and the traditional
separation between agriculture and forestry. Also, multiple cropping is
associated with unmechanized farming and low productivity; research in
intercropping and multiple cropping should be geared to increasing the
productivity and returns in both arable crops and forest products. The peasant
farmers' system of agro-forestry should be improved upon, and researchers should
evolve a combination of arable crops and fast-growing trees that can be easily
adapted by smallholder farmers.

The traditional cropping systems will continue until an alternative is
evolved that can fit into present technology, environmental constraints, and at
the same time maintain high crop yield. My feeling is that agro-forestry
research has the potential of offering an early and viable alternative.

Introduction

Farming systems can be defined as the distribution of plants and animals in
space and time and the combination of inputs believed to give maximum production
in socioeconomic, political, and cultural contexts.

In Africa, a farmer or farm family usually operates a small, diversified
agricultural enterprise. According to Okigbo (1978), farmers with a homestead in
an upland, well-drained soil may operate a compound farm or garden close to
their homestead and maintain two or more plots in cropping systems involving
natural or planted fallow and in the flood plain of a nearby river or steam.
They may also keep pigs, goats, sheep, and poultry for manure, meat, sales,
etc.; they may be palm wine tappers, basket-makers, musicians, or priests in the
traditional religion. The cropping mixtures on the farms often involve major
staples, vegetables, and condiments in multiple, double, relay, and patch
intercropping patterns of annuals, perennials, or both. The compound farm or
homestead garden usually carries more species of cultivated plants than bush
fallow farms.

The most common tradition in African cropping systems is the spatial
arrangement of crops on the field. The crops are established haphazardly in
mixed culture (Okigbo and Greenland 1975), the objectives being to take
advantage of local topographic features and micro-relief; disperse species at
wide enough spacings so that they do not compete for nutrients and light; ensure
that crop cover is adequate to control soil erosion and weeds; and ensure that
each species's requirements for sunlight are met. Where annual staples are
uniformly planted among tree crops, heavy pruning of the tree crops is usually
carried out to ensure that adequate light reaches the ground level. Whether
crops are grown on mounds, beds, ridges, or the flat, their spatial arrangement
and frequency in mixtures usually indicate their importance in the diet and
sometimes their uses.

Intercropping and Multiple Cropping

The simultaneous cultivation of different crops on the same piece of land has
been described interchangeably as mixed cropping or intercropping by Webster and
Wilson (1966) and Norman (1971). Ruthenberg (1976), however, distinguishes
between mixed cropping and intercropping on the basis of the pattern of the
intermixture.

The term intercropping has been used rather generally in the literature as
referring to the practice of growing two or more crops simultaneously in
different but proximate stands (Okigbo 1978). Grimes (1963) defined a common
practice in intercropping: the system of growing different crops in alternate
rows, which he terms alternate row cropping. Row intercropping is common in
filled areas, annuals often being planted under perennials. For instance,
tall-growing crops such as cassava or bananas are planted in young coffee,
cocoa, or rubber plantations (Sanchez 1979).

In multiple cropping, Herrera and Harwood (1973) indicated that each of the
crop mixture patterns has different physiological characteristics and different
advantages. For example, Norman (1974) showed that although there were at least
156 crop mixtures and many different spatial arrangements among Hausa farmers
near Zaria (Nigeria), the most popular arrangement was a systematic spatial
pattern on ridges. With intensification of cropping, interactions among plants
become critical. The most widespread multiple cropping systems practiced in the
humid tropics are mixed intercropping and relay intercropping.

Mixed intercropping is common when cereals, grain legumes, and root crops are
grown together and when little or no tillage is practiced. For example, farmers
in southern Nigeria plant simultaneously maize, cassava, vegetables, and
cocoyam. In Abakaliki, Nigeria, mixed cropping is practiced in mounds or ridges
of soil constructed with hoes. Several crops are planted on different parts of
the mounds. For example, an Abakaliki farmer plants yams on the mound, rice in
the furrow, and maize, okra, melon, and cassava on the lower parts of the mound.
Mounding is beneficial because it increases the volume of soil available to root
crops.

Relay intercropping is a practice where a second crop is planted after the
first crop has entered the reproductive growth phase but prior to harvest. A
common example is the maize-beans system used in most of Central America and
much of tropical South America. Maize is planted in rows, usually at the
beginning of the rainy season; when the ears are well formed, farmers break the
stalks just below the ear and plant climbing bean varieties. Relay intercropping
is also very common in rice-based systems in Taiwan. Up to five crops per year
can be harvested by two relay successions, rice-melons followed by rice again
relayed with cabbage and maize. At present, the maize-cassava relay is being
developed at the University of Ibadan, with researchers studying the effects on
soil nutrients.

Advantages and Disadvantages

Baker and Yusuf (1976) wrote that the almost universal practice of
traditional cropping systems by subsistence farmers throughout the world is an
indication that the system has evolved naturally as an answer to the challenging
environment.

The rationales for crop mixtures are that they may be relatively more
profitable than sole cropping (Chandra 1978), the difference between the
marginal value product of resources and the opportunity cost of the resources
being insignificant (Norman 1974); they are consistent with the goals of
security and year-round subsistence needs (Andrew 1972); they may alleviate
adverse conditions in the ecosystem; and they may maximize the space, water, and
nutrients available. Some of these benefits can be further promoted by good
tillage practices, based on the principle of minimizing disturbance of the
ground and vegetative cover. The practice of minimum tillage, with ample crop
residues left on the soil surface, has great potential.

Although monocropping tends to attract fewer diseases and insects, these are
more likely to be highly prevalent and to cause considerable damage. Cropping
mixtures may reduce the abilities of pests and diseases to spread. For instance,
interplanting has been shown to reduce insect problems in groundnut-sorghum and
cassava-maize mixtures and has reduced the incidence of bacterial blight.

The denser plant population usually found in crop mixtures may also help
control weeds (FAO 1968). In addition, because crops mature at different times,
mixtures may extend the period of the year during which the soil is protected by
leaf cover and root systems (Igbozurike 1971).

The disadvantages of traditional systems are that there is reduced yield of
the component crops (Chandra 1978; Webster and Wilson 1966; Agboola and Fayemi
1972); there may be competition for light, nutrients, and water (Dalal 1974;
Willey 1979; Webster and Wilson 1966); there may be allelopathic effects due to
excretion of toxic substances by one or more crops (Dalal 1974); the practice is
not well suited to modern agriculture or mechanization and, thus, research on
traditional systems has been inadequate (Ahmed and Gunasena 1979); and suitable
methods for investigation are difficult to define (Haizel 1974).

Research

Unfortunately, research workers appear to have been hesitant to tackle
multiple cropping experiments because of the many crop combinations in use, and
because multiple cropping by the peasant farmer is associated with
non-mechanical farming and low levels of productivity. Doubts have been
expressed as to whether any of the positive benefits of multiple cropping can be
exploited at more advanced levels of farming. Attempts to improve production by
the application of technology developed in temperate cropping systems have
failed in Nigeria and in most other tropical countries, not because of farmers'
conservatism but because the approach is inappropriate.

Farmers in the tropics have been noted to grow complex crop mixtures on
compound farms, especially in the rainforest zones, where staples, vegetables,
and perennial fruit trees are interplanted. As early as 45 years ago, Leakey
(1934) observed that the relay and mixed cropping practices had many obvious
advantages, and he recommended that those involved in agricultural development
should give serious attention to research on traditional food production
systems, especially those involving intercropping. This recommendation has
largely been ignored, despite the fact that peasant farmers have repeatedly
refused to adopt monocropping practices recommended by extension agents.

The main objectives of research into the productivity of mixtures might be to
screen mixtures for high-yielding combinations; to test alleged advantages of
traditionally grown mixtures; and to gain an understanding of the processes that
lead to advantages so that, in a specific environment, a rational choice of
components may lead to higher yields than are possible in monocultures.

Sturdy (1939) noted in East Africa that intercropping Crotalaria with millet,
and groundnuts with sorghum, helped in the maintenance of soil fertility.
Lambers (1940) reported that coffee intercropped with bananas in Kuri provided a
mulch that improved the fertility of the soil. Results of experiments carried
out by Agboola and Fayemi (1972) showed that legumes intercropped with early
maize gave a maize yield equal to that obtainable with 55 kg/ha of nitrogen
supplied as mineral fertilizer.

Much work still has to be done to quantify the nutrient level of soils under
traditional crop combinations. In a study conducted in 1981 composite soil
samples were taken from eight farms and their adjoining fallows, and in only one
case was the fallow land significantly higher in nutrient status than the
adjacent cultivated land. P, K, Fe, Zn, and Ca levels in the top 0-15 cm layer
were higher in the cultivated fields than in the adjoining fallows, and this was
attributed to the effect of burning after clearing. These data support the view
that the length of the fallow has been drastically reduced, thereby reducing
also the nutrient build-up. They imply that most farmers are actually cropping
infertile lands and that the nutrient status of the field does not influence the
farmers" choice of crop combinations. The choice of crop combinations seems
instead to be influenced by the food staples usually planted in the area, and
the inclusion of vegetables is related to the economic value of the crops and
food preferences. No definite effect of the crop mixtures on the soil nutrient
status could be established.

Summary and Conclusions

The best cropping system in the tropics, once the soil is considered as the
main factor sustaining crop production, is one that will not expose the soil to
erosion hazards Therefore, more research is needed to identify better
combinations, including agro-forestry schemes, so that the best type of rotation
can be developed for each ecological zone. Although zero tillage has been
advocated by IITA, this cannot be practiced ail over the humid tropics because
it is dependent on the soil, particularly the clay content, and the prevailing
weather conditions. In some areas, the temperature is low during the harmattan,
and the Fadama soils cake, necessitating extra soil preparation in the following
rainy season. Thus at the University of If e zero tillage has proved to be
unsatisfactory.

Besides protecting against erosion, an appropriate cropping system must guard
against the breakdown of the soil structure and nutrients. A good system needs
to take advantage of the tonnes of human, plant, and animal refuse being
produced. It should be emphasized, however, that traditional farmers will
continue with their existing system until an alternative is found that will
maintain higher yields, conserve the soil, control weeds, and generally fit into
their present technology.

Acknowledgements

The author appreciates the contributions from J.M.A. Tocunana and C.F.
Yamoah, both postgraduate students of the University of Ibadan's Department of
Agronomy.